Hydrogen detector with temperature sensing and control means



Oct. 29, 1968 s. c. LAWRENCE, JR 3,408,129

HYDROGEN DETECTOR WITH TEMPERATURE SENSING AND CONTROL MEANS OriginalFiled Jan. 19, 1961 3 Sheets-Sheet 1 IMPfE ME A51. E

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QTTOIQ/VE Y 1968 s. c. LAWRENCE, JR 3,403,129

HYDROGEN DETECTOR WITH TEMPERATURE SENSING AND CONTROL MEANS OriginalFiled Jan. 19, 1961 3 Sheets-Sheet 2 mm 4/4 4 cueaewr 44 Q /5752 EM/S5/0/1/ Ad/ M2 ea les/v7- /aa/fl ME 7252 6004 //V6' F525. :7. 99 57571445 50 755 r 4 xawa 0 W52 \FUPPL Y FIG, 1!.

INVENTOR.

FIG. 5: BY

Oct. 29, 1968 s. c. LAWRENCE, JR 3,408,129

HYDROGEN DETECTOR WITH TEMPERATURE SENSING AND CONTROL MEANS 3Sheets-Sheet 5 Original Filed Jan. 19, 1961 United States Patent' Office3,408,129 Patented Oct. 29, 1968 3,408,129 HYDROGEN DETECTOR WITHTEMPERATURE SENSING AND CONTROL MEANS Samuel C. Lawrence, Jr., 1814 S.142nd Place,

Seattle, Wash. 98168 Continuation of application Ser. No. 83,704, Jan.19, 1961. This application May 24,1966, Ser. No. 559,655

Claims. (Cl. 316-4) I ABSTRACT on THE DISCLOSURE This is concerned withsystems employing electron discharge tubes for measuring the hydrogeneffusion properties offluids. Hydrogen eifuses from the fluid into thetube through its wall. Some of the hydrogen is captured by the wall. Thetube is heated to an elevated temperature in order to expel the capturedhydrogen from the wall before the next use and the temperature iscontrolled during use. An electron current flowing in the tube is usedto measure the amount of hydrogen that has entered the tube duringsuccessive exposures to hydrogen etfusing fluids.

This application is a continuation of my prior patent application Ser.No. 83,704, filed Jan. 19, 1961.

This invention relates to improvements in electron discharge devicesemployed for measuring the effusion properties of a fluid into a solidobject.

It is well known that many 'metals, especially steel, are embrittled byvirtue ofhydrogen contained in them. The phenomenon resulting in suchembrittlement is called hydrogen embrittlement. Whether such gas ispresent "in molecular form or atomic form or both is still undetermined.Through there may be some question as to the form in which the hydrogenexists in the metal, the hydrogen that is present there may be referredto as dissolved or absorbed hydrogen. I

Hydrogen that causes embrittlement of metal may enter the metal invarious ways. For example, hydrogen may enter a piece of metal while thesurface of the metal is being cleaned with a paint solvent. Hydrogenresponsible for embrittlement may also enter metal during the course ofoxidation of the metal surface that occurs while the metal is exposed toa humid atmosphere for a prolonged period. Such embrittlement reducesthe strength and hence the life of any object made from such steel.

The rate at which hydrogen can diffuse from a fluid into a metallicobject can be measured to some degree of accuracy by submerging anelectron discharge device, often referred to hereinafter simply as atube, or vacuum tube, or electron discharge tube, or electronic tube, inthe body of the fluid and then determining the effect that suchimmersion has on the electronic characteristics of the tube. Phenomenaof these types have previously been reported. See, for example,Diffusion of Hydrogen from Water through Steel, by Francis J. Norton,Journal of Applied Physics, vol. 11, pp. 2621f, April 1940. See alsoUnited States Patent No. 2,526,038, issued to Herbert Nelson; UnitedStatesPatent No. 2,790,324, issued to Maynard A. Babb; and United StatesPatent No. 2,921,- 210, issued to Edward Schaschl, et al.

In such prior art devices, the electronic tube has been in the form of adiode, a triode, or a tetrode. Regardless of differences in structurebetween tubes, in accordance with the accepted theory of operation, thepartial pressure of hydrogen within the envelope of the tube isincreased while the tube is immersed in thefluid under investigation.This increase in pressure may be attributed to the migration of hydrogenions through the wall of the tube shell to the interior surface thereofwhere the hydrogen ions combine with electrons in the tube wall to formhydrogen gas. The rate of diffusion depends not only on the diffusionand desorption characteristics of the wall but also upon the effusionproperties of the fluid in which the tube is immersed. Since theeffusion property depends upon the fact that the fluid is in contactwith the wall of a metal object, it is sometimes referred to hereinafteras the hydrogen-effusion-into-metal characteristic of the fluid.

In utilizing such an electron discharge device, electrons emitted from athermally emissive cathode are accelerated toward an acceleratorelectrode to a potential that exceeds the ionization potential ofhydrogen. Accordingly, hydrogen gas bombarded by the electrons isionized. A portion of the positive ions so formed reach the collectorelectrode where they collect their missing electrons. The resultantelectron current flowing to the collector electrode from an externalcircuit is a measure of the pressure of the hydrogen atmosphere withinthe envelope of the electron discharge device. It is found that thecurrent so measured depends not only on the pressure of the hydrogen gasin the envelope but also on the temperature of the envelope. In fact, itis found that the current is very sensitive to temperature, that is, itincreases with temperature at a very high rate. Tests have shown thatunder many conditions, at least, the current (I) is related to thetemperature (T) by a formula of the following type:

in which n has a value of .about 6 and where I and T are constants andwhere K20.

The fact that the current increases as about the sixth power of thechange in temperature may be explained, in part, by the fact that whenions are present within the envelope in the same space with hydrogenmolecules and hydrogen atoms, the. forces between the various particlesdeparts greatly from that existing when the hydrogen atmosphere is freeof ions and electrons and is composed entirely of hydrogen molecules.Possibly, too, the rapid variation of current with temperature may beattributed partly to the effects of temperature on the absorpion ordesorption properties of the envelope of the tube as its temperature iselevated. The increased ion current may also be due to an increase inthe rate at which hydrogen diffuses through the shell into the tube asthe temperature is increased. Regardless .of the explanation of thephenomenon, the simple fact is that the current is very sensitive to thetemperature of the tube wall.

Accordingly, one of the objects of this invention is to provide anelectron discharge device with means for measuring the temperature ofthe wall so that the temperature may be known at the time ofmeasurement, and so that, if desired, the temperature of the wall may be.maintained at a predetermined value.

Another object of this invention is to provide an electron dischargedevice used as a hydrogen detector with means for closely regulating thetemperature of the wall in order that the temperature may be establishedat a pre- 3 3 determined .value, thereby eliminating erratic variationsin current readings that might otherwise result from changes in thetemperature of the tube wall and erratic variations in the rate at whichhydrogen gas is cleaned up by a getter, if present.

Still another object is to provide an improved arrangement forcontrolling the permeation rate by controlling temperature.

In this invention, a temperature-sensitive element is mounted in heatexchange relationship with the wall of an electron discharge tube inorder to measure its temperature during operation, and in the bestembodiment of the invention, heating or cooling elements are mounted inheat exchange relationship with the envelope of the tube in order tocontrolthe temperature. In this way, the irregularities in measurementsof hydrogen effusion that might otherwise be produced because of'uncontrolled or unknown temperature conditions are reduced oreliminated.

The foregoing and other objects of the invention, together with variousfeatures and advantages thereof will be understood from the reading ofthe following description, taken in connection with the accompanyingdrawings, wherein:

FIGURE 1 is a schematic diagram of a system employing this invention forthe measurement of the hydrogeneffusion-into-metal properties of afluid;

FIGURE 2 is a schematic diagram of an alternative system;

FIGURE 3 is a diagram showing how the invention is utilized to measurethe hydrogen-effusion properties of a liquid;

FIGURE 4 is a wiring diagram of a circuit of the type that may beemployed for measuring the ionization current produced in the tube ofFIGURE 1;

FIGURE 5 is a schematic diagram of an alternate embodiment of theinvention;

FIGURE 6 is a schematic diagram of another alternate embodiment of theinvention;

FIGURES 7, 8, 9 and 10 are graphs employed to explain some of thephenomena involved in the operation of the hydrogen detector of thisinvention; and

FIGURE 11 is a schematic diagram of a type of tube that may be employedin this invention.

In FIGS. 1 and 2, there is shown a standard vacuum tube 10 known as a6V6 tube 10 which has been modified for use with the present invention.This vacuum tube is provided with an envelope 11 which comprises a metalshell 12 sealed to an insulating base 14. The shell 12 is formed of ametal such as steel which is permeable to hydrogen. Such a tubecomprises an indirectly heated cathode 16, inner and outer grids 18 and20, and a plate 22. All of the electrodes are of cylindricalconfiguration and they are supported concentrically within the envelope11. Each of the electrodes 16, 18, 20 and 22 and the shell 12 isconnected electrically with an external metallic prong-shaped terminal15. The two terminals of the heater 24 mounted in contact with thecathode 16 are also elec trically connected to two external terminalprongs.

In the conventional method of manufacturing such a tube, the envelope 11is evacuated by means of a vacuum pump and then sealed off against theingress of air. At the time of sealing, the pressure within the envelopemay be about 10* or 10 mm. Hg. In order to improve the operation of sucha tube, the interior space is further evacuated by the evaporation of acharge 13 of gettering material within the envelope. Such a getteringmaterial may, for example, consist of barium salts in combination withsalts of aluminum or beryllium which when evaporated (fiashed) producefree barium, or other material capable of absorbing residual gasremaining in the envelope after sealing. Upon evaporation, suchgettering material forms a localized deposit on the interior wall of thetube, such as the deposit 26 shown at the lower end of the tube 10 ofFIG. 1.

As is Well known,-such gettering material absorbs residual gasescontained within the envelope of such a vacuum tube, thereby reducingthe gas pressure to a much lower value, such as to a pressure of 10" mm.Hg. In some cases, the deposit of gettering material is at the upper endof the tube, instead of at the lower end, as shown. In other cases, thedeposit of gettering material is on the side of the tube. In any event,during the course of manufacture of a series of tubes, the areacoveredby the deposit 26 of gettering material varies in a rather irregularmanner from one tube to another. On the other hand, it is also possibleto produce such high vacuums with special pumps or'with getter'sithatoperate only when turned on as with special auxiliary filaments. In suchcases, the masking of gettering deposit (masking) .on thetube surface isnot necessary, but windows to control differencesin gas permeation ratesdue'tovariations in shell structure or composition are required in orderto give satisfactory results.

The outer wall of the shell 12 is coated with a hydrogenimpermeablelayer 3Q over a portion of the external surface thereof, but-leaving arestricted portion of the she'll free of such coating material, thusforming a hydrogenpermeable, or hydrogen-pervious, window 34; The layeron thecoated portion of the shell thus forms a barrier to the flow ofhydrogen into the interior of the tube, while the uncoated portion formsa hydrogen-permeable window which permits the flow. of hydrogen into theinterior of: the tube through the window 34.

In the best mode of practicing the present-invention now known, atemperature-sensing element such as a thermocouple 35 is mounted incontact with the interior surface of the shell 12. The two conductors36-36 of the thermocouple are connected to two of the prong-shapedterminals 15. Two lead wires connected to these terminals lead to aregulator unit wh'ch is uti ized to regulate the temperature of theshell. Such a regulator 40 responds to changes in voltage produced bythe thermocouple between the leads 38 and 39 when the temperature of theshell changes. In the present instance, this regulator is employed tocontrol a cooling system for maintaining the temperature of'the shellconstant. A sensitive voltage measuring circuit 41 including amillivoltmeter or microvoltmeter 410 connected across the two leads 38and 39 is employed 'to indicate the temperature.

In order to cool the shell to prevent the temperature from rising abovea predetermined amount, cooling pipes 42 encircle the shell 12 in heatexchange relationship therewith. The cooling pipe may be in the form ofa spiral arrangement located near the base 14 of the tube, though it mayalso be located elsewhere. Opposite ends of the cooling pipe 42 areconnected to pipes 43 that lead to a supply of cooling liquid containedin a liquid cooling system 44. The liquid may be supplied by arefrigeration system, or it may simply be supplied from water mains.

In" any event, the changes in voltage impressed upon the regulator 40by'changes in temperature detected by the thermocouple 35 are employedto regulate the cooling effect of the cooling water'flowing through thecooling pipe 42. This regulation may be by regulating the temperature ofliquid being circulated through a refrigerator. On the other hand, theregulating system 44 may simply be in the form of an automatic valvewhich regulates the rate at which cooling water of fixed temperatureflows through the cooling pipe. In either event, the cooling system iscontrolled bythe adjustable regulator 40 in response to the response ofthe thermocouple to increase the cooling effect of cooling water flowingto the cooling pipe when "the temperature of the shell rises above apredetermined temperature or to decrease the cooling effect when thetemperature of the shell falls below that temperature. An adjustableelement indicated by the control element 46 in the regulator may beemployed to set the temperature at any predetermined value.

To further. enhance the effectiveness of the hydrogen detector of thisinvention, electric heating coils 50 are wrapped around the base 14. Byflowing current through these heating coils, accumulation of condensedmoisture around the prongs is prevented. In this way, errors that mightotherwise be introduced by the presence of such moisture are avoided.Such errors arise partly because of hydrogen effusion from water throughthe base and partly because of the increased conductivity that a film ofmoisture can introduce between prongs. v

In an alternate form of the invention, as shown in FIG. 6, thetemperature is regulated by means of wire heating coils 42a instead ofby the use of liquid cooling coils 42. Thus, in the modification of theinvention illustrated in FIG. 6, heating wires 42a encircling theenvelope 11 in heat exchange relationship therewith are suppliedelectric power from a source through an adjustable temperaturecontroller 40 which is connected to the thermocouple 35 through theconductors 38 and 39;

It will be noted that in both of the embodiments of the inventionillustrated in FIGS. 1 and 5, the temperature regulating tubes 42 are inproximate heat exchange relationship with the shell while the heatingelement 50 is in proximate heat exchange relationship with said base.With this arrangement the respective elements produce maximum cooling orheating effects respectively on the shell and the base.

In the system illustrated in FIG. 1, whenever the temperature of theshell 12 rises above a predetermined value, the regulator 40 causes thecooling water flowing through the tube 42 to absorb heat from the shell.This may be done either by flowing the water through more rapidly thusopposing the increase in temperature of the shell or else causes thewater to flow through at a lower temperature thus likewise opposing therise in temperature of the shell. Which method is employed'depends onthe nature of the heat regulating system. Alternately, instead ofemploying cooling water flowing through the tubes 42, heating watercould be employed. In this case, the liquid cooling system 44 respondsto a rise in temperature as sensed by the thermocouple 35 to reduce thetemperature of the heating water or else to reduce its rate of flow soas to counteract the increase in temperature of the envelope.

In the system of FIG. 6, in a similar way, when the temperature of theshell is increased, as indicated by the response of thermocouple 35, therate of flow of heat from the wire coil 42a to the envelope is decreasedand when the temperature of the shell is reduced, as indicated by theresponse of the thermocouple, the rate of supply of heat from the wirecoil 42a to the envelope is increased. In any event, the thermocouple 35is employed to control a heat absorbing system of the type shown in FIG.1 or a heating supply system of the type shown in FIG. 6 in order tomaintain the temperature of the shell nearly constant.

In some cases it may be desirable .to apply a thermocouple to theexternal surface of the shell, as indicated in FIG. 5. In this case, theleads 36a, 36a of the thermocouple may be connected directly to theregulator. Such an arrangement is especially desirable when use is madeof a commercially available tube.

However, in the most effective arrangement, the thermocouple is mountedon the interior surface, as shown in FIG. 1. Such an internal mountingof the thermocouple provides a more accurate determination of thetemperature of that part of the shell which is in direct communicationwith the atmosphere contained within the envelope. In order to enhancereproducibility of measurements, the thermocouple junction is mounted ona portion of the wall through which the hydrogen diffuses, that'is atthe window 34 if one is present. v

In the most effective forms of the invention the temperature isregulated automatically. In somewhat less reliable but less costlysystem, the temperature is controlled by water flowing from the buildingmains through a pipe 43a in heat exchange relation with the shell asshown in FIG. 2. In this case the temperature is regulated 6 byadjusting the flow rate by manual manipulation of the valve V.

Frointhe foregoing description, it will be apparent that closetemperature regulation for the purpose of maintaining the temperature ofthe wall of the electron discharge device has been provided.

In order to facilitate the making of reliable comparative measurementswith different tubes, the walls of the tubes are partially coated withhydrogen-impervious material, such as epoxy resin, leaving an area ofthe shell uncoated. In this way, a hydrogen-permeable window 34 isformed in the shell. By suitable adjustment of the sizes of suchwindows, whether 'by the addition or removal of such coating material,all of the tubes of a given set may be made to have a predeterminedsensitivity at a predetermined temperature.

More particularly, since different tubes may be used for makingcomparative tests, and since test results must be of uniformlyrepeatable accuracy, it is necessary that all tubes of a series mustpossess a predetermined standard of hydrogen permeability andsensitivity at a predetermined temperature. Therefore, in preparing acoated tube carrying, for example, 'a layer of the hydrogen-impermeableepoxy resin, and employing a thermocouple and cooling system, the tubeis tested under standard operating conditions in a fluid of knownproperties to be sure that it has the required degree of hydrogenpermeability at a predetermined temperature. If necessary, the area ofthe uncoated portion of the shell is increased or reduced, as may beneeded, to establish equality of sensitivity of different tubes at thesame temperature. But for tubes where it is not convenient to change thewindow area, or where suflicient change in permeability of the tubeshell cannot be achieved efliciently by changing the window dimensions,greater changes in permeability can be achieved by varying the shelltemperature. For example, a change in shell temperature of only 144 C.will result in a change in permeation rate of hydrogen (or any other gasin any other metal, for that matter) by a factor of 2 that is 1024.

From the standpoint of the preparation of the metal surface of theenvelope 12, either for proper diffusion of the gas, or for applicationof the hydrogen-impervious resin, the metal tube is cleaned to removeany paint coat which may have been applied by the manufacturer, or toremove oxidation products, or the like.

This cleaning process may include fine sand blasting, electropolishingfor a few minutes with a sulphuric acid, glycerol solution, waterrinsing, and a 5-second dip in a 6% hydrochloric acid solution tobrighten and activate the metal surface, all followed by a distilledwater rinsing, followed by acetone rinsing or spraying. In theseoperations, the tube base 14 and the joint between the base and themetal shell 12 are masked and sealed to protect them from moisture andfrom vapor.

Following coating of the desired proportion of the metal tube with thedesired impervious resin, the latter is heat-cured or otherwise suitablycured. If, on test, after curing, too much resin is found to have beenapplied, appropriate portions may be stripped away or otherwise suitablyremoved. 1

In the above manner, each tube may be appropriately cleaned and paintedor otherwise partially coated so that all tubes possess the desiredstandard of sensitivity of hydrogen diffusion rate at a giventemperature.

In FIGS. 2 and 3, there is shown schematically an arrangement formeasuring the hydrogen effusion properties of a liquid. In this case,the end of the shell 12 of the probe 10 is located beneath the mainlevel 37 of the liquid under investigation, while the insulating base 14is located above that surface. An electric cable into which theterminals 15 have been plugged connects the probe 10 with a measuringcircuit 50a. This circuit 50a includes a first meter M for measuring acharacteristic of the tube 10 that depends upon the amount of hydrogenthat has flowed into the space within the envelope of the tube throughthe window 34, and a second meter M that is used for standardizing theelectron emission of the cathode.

By making measurements of the hydrogen effusion properties of differentliquids, information is thus'obtained for monitoring the operations of asystem in which metallic objects are treated with such liquids. By useof such a tube, liquids which have the lowesthydrogeneifusion-into-metal characteristics may be selected. Anyhydrogen embrittlement of metallic objects treated by liquids can thusbe minimized. For example, a' series of tubes of equal sensitivity mayall be painted with a common paint that is to be removed. Then each ofthese tubes may be partially submerged in aditferent liquid paintremover in the manner just described and the effect of the differentpaint removers on the different characteristics of the respective tubesmeasured. In this way, the differences in the hydrogen diffusionproperties of the different paint removers may be ascertained. For besteffects, the tests are all made at the same temperature. At least allthe tubes should be at the same temperature. This invention makes itpossible to accurately measure and to accurately control the temperatureof the tubes during use.

A measuring circuit of the type that may be employed for measuring thepressure of the hydrogen atmosphere formed within the envelope 11 of thetube of this invention is shown in FIG. 4. As indicated there, thecathode 16 is connected to one end of a potentiometer 51, the other endof which is connected to the negative terminal of a power supply PS. Theinner grid 18 is connected to the slide wire 52 of the potentiometer.The outer grid 20 is connected through a meter M to the positiveterminal of the power supply PS, and the plate 22 is connected through amicro-microammeter M to the negative terminal of the power supply PS.The voltage supplied by the power supply PS is of such a magnitude thatelectrons accelerated from the cathode 16 toward the plate 22 attainenergies corresponding to those above the ionization potential ofmolecular hydrogen. In use the shell 12 is connected to anotherelectrode such as the cathode 16.

The outer screen 20 is employed as an accelerator electrode. The plate22 is employed as a positive charge collector, or positive ioncollector. The inner grid 18 is employed for regulating the electroncurrent formed within the tube under standard conditions. Bymanipulating the slider 52 on the potentiometer 51, the current flowingthrough the tube at any time may be standardized, thus compensating fordifferences in the electron emissive properties of cathodes 16 ofdifferent tubes, or for compensating for differences in the electronemissive properties of the cathode of any tube during the life of thetube. The effectiveness of the inner grid for this purpose arises fromthe fact that the 6V6 tube has a gradual, or remote, cut-offcharacteristic as distinguished from a sharp cut-off characteristic thuspermitting a gradual change of current to be produced when the bias onthe inner grid 18 relative to the cathode 16 is changed. The bias on theemission control grid may also be adjusted when the probe is in use inorder to adjust its sensitivity. Over a wide range of operation the ioncurrent indicated by meter M is proportional to the emission currentindicated by meter M In operation, hydrogen effusing from the liquiddiffuses through the window 34 of the tube 10 to the inner surfacethereof. At the inner surface the hydrogen is desorbed thus increasingthe pressure of hydrogen gas existing within the envelope 11. Asmentioned above, the hydrogen may flow through the wall in the form of apositive ion current, combining somehow with electrons on the innersurface of the envelope, thereby forming atomic hydrogen. Such atoms ofhydrogen then combine within the envelope, probably at the surface, toform molecular hydrogen which thereby establishes a moleculat hydrogenatmosphere within the envelope. Regardess of the explanation of thephenomena involved, the fact is that the pressure of hydrogen gas withinthe envelope is increased when the tube is immersed in a liquid which iscapable of causing such diffusion of hydrogen into the envelope/Bylocating the window at a distance from the gettering material, directabsorption of hydrogen by gettering material as the hydrogen diffusesthrough the shell is avoided. Instead, the hydrogen is desorbed rapidlyfrom the portion of the wall free of gettering material, thus maximizingthe rate of flow of hydrogen into the space within the probe envelope.

In the process of accelerating electrons from the cathode 16 toward theaccelerator grid 20, electrons travel at a high speed through the spacebetween the cathode 16 and the accelerator grid 20. Thereafter, they aredecelerated in the space between the accelerator grid 20 and thecollector plate 22. Electrons bombard hydrogen in the space between theaccelerator grid 20 and the plate 22 thereby ionizing the hydrogen gas.As a result, electrons represented by the symbolr and hydrogen ionsrepresented by the symbols H+ and H and H are formed in the space withinthe envelope between the accelerator grid 20 and the collector plate 22.Such hydrogen ions, being positively charged, are repelled by theaccelerator grid 20 toward the collector plate 22. When they strike thecollector plate, they collect their missing electrons which thereforeflow through the micro-microammeter M At the same time, electrons formedin the ionization process are drawn toward the accelerator grid 20.These electrons flow to the positive terminal of the power supply.Hydrogen ions and electrons are also formed in the space between the twogrids by virtue of the bombardment of hydrogen gas in this region by theaccelerated electrons. These hydrogen ions flow to the inner grid 18,,where they are discharged, and these electrons flow to the outer grid20. The latter hydrogen ions and electrons do not contribute to thecurrent flowing through the micro-microammeter M In practice, therefore,the magnitude of the current flowing through the meter M is a measure ofthe pressure of hydrogen gas present within the envelope 11 at any time.In practice, it is observed that when a probe 10 exposed to fluid isfirst turned on, the magnitude of the current flowing through the meterM changes as a function of time. For this reason, measurements are madeafter the current has become stabilized, or else has fallen below somepredetermined value. Then the probe is mersed. in the fluid under testand the rate at which the ion current increases is measured while theprobe is exposed to the fluid.

In normal usage, when a probe is first energized the ion current risesrapidly to a high peak value which may exceed 10- amp. This currentarises from the fact that the initial heating of the probe, especiallythe initial heating of the cathode, causes some of the gases that havepreviously been absorbedon various electrodes and the internal surfaceof the shell to be desorbed. While the probe remains warm these gasesare absorbed by the gettering material gradually reducing the ioncurrent to a value of 4X10 amp. or. less. The time required for the ioncurrent to reach-such a sufficiently low value to permit subsequentsignificant measurements to be made varies between 10 to 30 minutes, ifthe tube has once been previously properly prepared.

In FIG. 7 a series of graphs is shown which represent in a general waythe manner in which the ion current varies as a function of time atdifferent gas pressures while the temperature is constant. Here it willbe noted that in all cases the ion current increases fairly rapidly atthe inception of operation but that gradually the current reaches asaturation value which depends to a large extent on the ultimatepressure of the gas in the tube. For most satisfactory results the ioncurrent is measured at a predetermined time T after the inception ofpermeation and long before saturation is reached. In this way ameasurement is obtained which depends on the permeation rate.Alternativelymeasurements are made at two such times and the rate ofchange of ion current determined therefrom. It is to be noted that asthe pressure of the hydrogengas increasesthe saturation value of the ioncurrent also increases. The rate at which the saturation value isreached depends on the rate of permeation of hydrogen into the tube andthe gettering rate of the getter, if one is used, or on the gas-cleanupspeed of any other gas-cleanup mechanism used with the tube. Each of thegraphs of FIG. 7 corresponds to'measurements obtained with a differenttype liquid.

. FIG. 8 is a graph that shows how the ion current varies in proportionto the pressure of hydrogen within the tube. The measurement of FIGS. 7and 8 represent the increase in ion current caused by the presence ofhydrogen. For example, if the ion current without hydrogen permeation is1 10- amp. and the total ion current when hydrogen is permeating thewall is 3 1O- amp., then the effective ion current is 2X10" amp. Exceptin the case of FIG. 9, it is the effective ion current that is plottedin the various graphs.

When a tube is not in use, some of the hydrogen gas within the tubebecomes adsorbed by the gettering material and some leaks outwardlythrough the Wall if the tube is in a hydrogen free atmosphere, thusrestoring the tube to a high vacuum condition. The tube may be restoredmore rapidly by maintaining it at an elevated temperature, say 190 C.When so restored, the tube may be used again. However, when the hydrogenabsorption capacity of the gettering material is exhausted, the pressureof the hydrogen atmosphere remains high and the tube calibration isgreatly altered, thus requiring replacement of the tube.

In one method of employing this invention, the shell 12 of the tube isheated to prepare it for reuse. In the heating process, hydrogen thathas previously been absorbed in the wall of the shell is driven out ofthe shell rapidly. Some of the hydrogen released in this process leaksoutwardly, some enters the tube. The rate of expulsion of hydrogen fromthe shell wall by this method is increased as the temperature is raised.In practice, with conventional gettering materials the temperature ofthe shell is raised to as high a point as possible without causingdeterioration or evaporation of the gettering material. With abarium-salt gettering material a suitable temperature is 190 C. l

The electrodes of the tube may also be maintained energized during therestoration process. Maintaining the tube at such an elevatedtemperature for about one hour is generally sufficient to restore thetube, so long as the gettering capacity of the getter has not beenexhausted. It is useful in such a restoration technique to 'make use ofthe thermocouple for indicating the temperature and in many cases, it isdesirable to employ one ofthe automatic temperature-regulating systemsdescribed, during tube restoration in order to prevent the temperaturefrom exceeding the limit mentioned above which deterioration orevaporation of the gettering material may occur.

In FIG. 9, two Graphs G and G show how the value of the ion currentvaries as a function of temperature in a probe not subjected to hydrogenpermeation. Graph G shows how the ion current varies with temperaturefor a hydrogen detection probe that does not contain argon or othernoble gas, while Graph G shows how the ion current varies withtemperature for a hydrogen detection probe that contains argon.Referring to FIG. 9, it will be noted that below the boiling point ofargon, that is below 80 K., the two graphs coincide, but above thistemperature, Graph G lies above Graph G Furthermore, Graph G shows thatthe contribution of argon to the ion current changes rapidly with thetemperature. For this reason, and because the ion current caused by anyhydrogen that is present during permeation doubles for each 14.4" intemperature, it is important that the temperature at which themeasurements of ion current are made be known and be accuratelycontrolled.

Graph G at the bottom of FIG. 9 represents the small ion current whichis produced by the ejection of photoelectr-ons from the collector plateduring operation. Such electron ejection is caused by bombardment of thecollector plate by gamma rays that are generated by the low energyelectrons striking the accelerator grid. It is to be noted that thephotocurrent does not vary with temperature.

In FIG. 10 there is shown'a graph representing how the pressure ofhydrogen within the tube varies as a function of time. In the first partG of the graph, the pressure rises gradually in an exponential manner asindicated in FIG. 9. Subsequently, if the tube is removed from exposureto the source of hydrogen at a time T, the hydrogen gas within the tubeis absorbed by the gettering material, thus resulting in an exponentialdiminution of pressure with, time as indicated by the part G of thegraph. In practice, it is often observed that deviations from theexponential decay characteristic of Graphs G occur, especially at thecommencement thereof. These deviations are believed to be due to thecontinued flow into the tube of the hydrogen absorbed within the shellwall, even though the external source of hydrogen has been removed.

Though the tube has been described above particularly with reference tothe use of getters that are deposited by evaporation on the interiorsurface of the shell, it will be understood that filament-type gettersmay also be employed. Such a getter may be in the form of a tantalumfilament 70 mounted within the tube, as shown schematically in FIG. 11.This filament like the operating electrodes of the tube are connected toprongs in the base of the tube. By the use of the temperature regulatingsystem of this invention the tube is rapidly restored to a standardtemperature suitable for making a test even though the tube may havebecome heated or cooled during the restoration process.

As mentioned above, an ion current represented by the Graph G flows eventhough no hydrogen is present within the tube. In the best method ofusing the detector this current is reduced to a minimum value byeliminating as much of the nongetterable gas as possible from the tubeduring manufacture. In order to achieve this result during themanufacture of the hydrogen detector tubes 10, the electrodes and thegetter charge are first mounted on the base, and the base is sealed tothe shell, then the envelope is flushed through an opening therein witha getterable gas that is substantially free of any nongetterablecomponents, then the envelope is evacuated to reduce the pressure of gaswithin it to a low value of less than about 10- mm. Hg, and then thecharge of gettering material originally mounted within the envelope isevaporated to form a deposit on the interior surface. A getterable gassuitable for use in flushing is one which is composed of componentsselected from the group consisting of hydrogen and nitrogen. Thenon-getterable gases to be avoided are the noble gases such as argon,neon, krypton and the like. Of these argon is the most likely to bepresent if precautions are not taken in accordance with this inventionbecause argon is the most common noble gas present in air. A gassuitable for use in flushing is, therefore, a getterable gas having amajor portion thereof composed of com onents selected from the groupconsisting of hydrogen and nitrogen and being substantially free ofnoble gases. By manufacturing tubes in this manner the zero ion currentcaused by the presence of nongetterable gas is reduced to a minimum.

In practice it is found that regardless of the cause, the permeationrate just about doubles when the temperature of the wall of the tuberises 14.4 C. In addition over many of the temperature ranges in whichsuch a probe 1 1 is used, nongetterable gases such as argon cause anincrease in ion current indicated by graph G that is not attributable tohydrogen. It is thus seen that for accurate measurements it is desirableto maintain the wall at a temperature that lies within a fraction of adegree of a predetermined value. While it is not always necessary thatthe temperature be known, it is necessary that temperature be controlledaccurately. By means of this invention variations in the ion currentthat would otherwise cause uncontrolled or undetermined changes intemperature between the time of calibration of the tube and the time ofmeasurement by means of the tube are greatly reduced or completelyeliminated. In the best method for using the invention the temperatureis controlled automatically. But in a simpler method of using theinvention, it is controlled manually.

In this specification the terms'effusion, diffusion, permeation, anddesorption have been employed to describe various phenomena that affectthe flow of hydrogen from abody of liquid through the shell of a probeinto the space within the shell. The effusion property refers to aproperty of the liquid. It represents the ability of the liquid tosupply hydrogen to the external surface of a probe or to the externalsurface of a solid object that is immersed in the liquid. This abilitymay be due to electrical characteristics, chemical characteristics, orothers. The term diffusion refers to the migration of hydrogen from onepoint to another within the material composing the shell of the probe orthe object. The term desorption refers to the ability of a surface tocause hydrogen contained within the wall or within the object to emergefrom the surface in gaseous form. The term permeation refers to theover-all ability of a wall member to permit the flow of gas through thewall from the space on one side thereof external to the wall to thespace on the other side thereof external to the wall. It is thus seenthat in the flow of hydrogen from the liquid under test into the spacewithin the shell of the probe, the hydrogen effuses from the liquidthrough the external surface of the shell into the body of the shell.There the hydrogen diffuses to the internal surface of the shell. Atthis point the hydrogen is desorbed thereby forming a gaseous atmospherewithin the shell. The permeability of the shell depends upon thediffusion characteristics of the shell material and also the desorptioncharacteristics of the internal surface, and also on the nature of theinteraction between the external surface and the fluid undergoinginvestigation.

While the tube of this invention may be used in many ways and with manycircuits, it is clear from the foregoing description that a novelhydrogen detection apparatus of greater reliability is provided by thisinvention. While the invention has been described with respect to onlycertain specific embodiments thereof, it will be understood that it maybe applied in many other ways. For example, though the invention hasbeen described as being applicable to a tetrode, it may also be employedwith triodes and even with diodes. Furthermore, while the invention hasbeen described with specific reference to the most important applicationthereof known, namely, to the measurement of the hydrogen effusioncharacteristics of liquids in which a tube is immersed, it will beunderstood that the invention is also applicable where the tube isimmersed in gaseous fluids or other hydrogen-bearing atmospheres. Itwill also be clear that by suitable modification of materials, it mayalso be employed for the measurement of gases other than hydrogen. It istherefore to be understood that the invention is not limited to thespecific embodiments of applications thereof described, but that it maybe embodied in many other forms, and that various other materials may beemployed, and that it may be used with other circuits and in otherenvironments than those specifically described herein.

I claim:

1. In a method for using a hydrogen-detector tube to measure thehydrogen effusion properties of fluids and for restoring such tube forreuse, said tube having electrodes within the tube for hydrogenmeasurement and havinga hydrogen-absorbing metal wall'the steps whichcomprise; immersing a portion of Said tube within a hydrogenelfusingfluid with the hydrogen-absorbing metal in contact with the fluid at apredetermined temperature below C. whereby hydrogen is etfu sed into thewall and some of the effused hydrogen is absorbed in the wall; removingsaid tube from said fluid; and expelling such absorbed hydrogen from thetube wall by restoration heating of the tube wall to an elevatedtemperature above said predetermined temperature, somehydrogen passinginto the interior of the tube from'said wall and some hydrogenpassingout of the tube from said wall. I 2. ,In a method as in claim 1,the steps of maintaining said electrodes energized during such heating,and measuring the temperature of the tube while being heated to suchelevated temperature.

3. In a method as in claim '2, the step of automatically regulating thetemperature of the tube during the restoration heating.

4. In a method as in claim '2, the steps of repeatedly restoring andreusing said tube by the method specified in claim 1, and wherein suchreuse involves'exposing said tube to a hydrogen effusing fluid only at atemperature lower than said elevated temperature whereby hydrogen isabsorbed in the wall of said tube during such exposure.

5. In a method as in claim 1, the step of automatically regulating thetemperature of the tube during restoration heating and measuring theregulated temperature.

6. In a method as in claim 1 wherein the tube contains a getteringmaterial and the restoration temperature is held at a temperature atwhich the gettering material removes hydrogen from the space within thetube.

7. In a method of measuring hydrogen effusion by means of a hydrogendetector tube having electrodes within the tube for measuring ioncurrent flowing between the electrodes in accordance with the amount ofhydrogen within the tube and having a hydrogen-absorbing metal wall, thesteps which comprise:

immersing a portion of said tube within a hydrogeneffusing fluid withthe hydrogen-absorbing metal in contact with the fluid at apredetermined temperature whereby hydrogen is eifused into the wall andsome of the eifused hydrogen is absorbed in the wall; removing said tubefrom said fluid;

expelling absorbed hydrogen from the tube wall by heating the wall to anelevated temperature above said predetermined temperature while removedfrom said fluid, whereby hydrogen is driven out of the wall of the tubeinto the interior of the tube;

again immersing a portion of said tube within a hydrogen-effusing fluidwith the hydrogen-absorbing metal in contact with the fluid only at apredetermined temperature below said elevated temperature wherebyhydrogen is again effused into the wall and some of the latter eflusedhydrogen is absorbed in the wall; removing said tube from last mentionedfluid;

and again expelling absorbed hydrogen from the tube wall by heating thewall to an elevated temperature above said predetermined temperaturewhile removed from said latter fluid whereby hydrogen is driven out ofthe wall of the tube. V

8. A method for measuring hydrogen effusion as defined in claim 7wherein maintaining said portion of the tube being immersed within ahydrogen effusing fluid in both cases sufiiciently long to. enablehydrogen effusing from the fluid to enter the space within the tube;

applying a voltage across said electrodes after hydrogen has efiusedthrough the wall of said tube into the interior thereof during eachimmersion;

13 14 and measuring the current flowing between the elec- ReferencesCited trodes to ascertain the amount of hydrogen that has UNITED STATESPATENTS flowed into said tube during each immersion.

9. A method as defined in claim 8 in which different 2921210 1/1960schaschl 324 33 X fluids are used in the successive immersions. 5 OTHERREFERENCES 10. A method of measuring hydrogen effusion as (16-Publication: Norton, F. 1., Journal of Applied Physics,

fined in claim 9 in which both immersions take place at 324/33, vol. 11,April 1940, Pages 262-267.

the same low temperature compared with said elevated temperature.RICHARD H. EANES, JR., Primary Exammer.

